Nutrient pulses after prescribed winter fires and preferential patterns of N uptake may contribute to the expansion of Brachypodium pinnatum (L.) P. Beauv. in highland grasslands.
Question: Do prescribed winter fires - authorized to prevent shrub encroachment and control necromass accumulation in protected grassland communities - promote significant changes in soils and in the pool of nutrients available to plants? Does Brachypodium pinnatum, an expanding native grass, take better advantage of the most dynamic nutrient pulse, that of N, than other dominant native grasses? Location: Aezkoa commons, western Pyrenees. Methods: We scheduled experimental winter burnings in three semi-natural grasslands with incipient shrub encroachment and tracked the short-term physical-chemical changes produced in the soil. During the peak biomass period, we performed a stable isotope field experiment in order to follow the fate of three different chemical forms of labelled N (15NO3-, 15NH4+ and 13C2-15N-glycine) in 48 undisturbed stands of B. pinnatum and Festuca gr. rubra. Results: The soil pools of available phosphorus, potassium and ammonium significantly increased after the fire. Four months later, a pulse of nitrate was also recorded. The isotopic labelling experiment revealed that, although the three forms of N were absorbed by the grasses, significant differences in 15N tissue enrichment appeared after 48 h. Altogether, the results indicated a higher response of B. pinnatum to the NH4+ pulse than F. rubra, although the latter was able to absorb intact glycine, unlike B. pinnatum. Conclusions: Controlled winter burnings, despite being low intensity, cause a significant release of the main nutrients associated with plant growth. This makes the plant community more susceptible to floristic changes. The field experiment suggests that B. pinnatum may make better use of the post-fire NH4+ pulse than F. rubra. This preferential uptake, together with other competing mechanisms of the species, may trigger the expansion of B. pinnatum in burned areas.